1. Field of the Invention
The present invention pertains to the field of electron beam tubes and more particularly to an electron gun header having an adjustable perveance that enables the cathode flux to be adjusted and optimized for improved beam focusing.
2. Description of Related Art
Electron guns are well known and used in standard travelling wave tubes (TWTs), mini-TWTs, klystrons, linear accelerators, and other radio-frequency (RF) electron devices. Such devices typically cause an electron beam originating from an electron gun to propagate through an evacuated tunnel or drift tube that includes an RF interaction structure. The electron beam must be focused by magnetic or electrostatic fields, or both, within the device to minimize beam loss by collision with the walls of the device itself. For example, a TWT operates as a broad-band microwave amplifier that relies on the interaction of a propagating RF wave with the propagating electron beam. In such a tube, the focused electron beam propagates with a velocity slightly faster than that of the RF wave such that the electrons may lose kinetic energy to the wave, thus amplifying its power. Controlling the focusing and propagation of the electron beam is thus important to the performance of the TWT.
In a device such as a TWT, the electron beam is formed by an electron gun, which typically comprises an electron-emitting cathode and an anode. The cathode is typically heated to enable thermionic electron emission. When the anode is raised to a potential that is positive with respect to the cathode, the electrons begin to flow as a beam. The geometry of the anode, the cathode, and other focusing electrodes create electromagnetic fields that define the path of the electron beam. In a Pierce gun configuration, the electron beam passes through an opening in the anode to enter the main body of the electron device. In other configurations, a grid is positioned in front of the cathode and affixed to the electrically isolated focus electrode. When the grid is pulsed to a potential sufficiently negative with respect to the cathode, it cuts off the electron current flow and can be used to create a modulated or pulsed electron beam.
Many electron guns are designed to exhibit a high perveance, which is defined as the ratio of the space-charge-limited beam current to the gun cathode-to-anode voltage raised to the three halves power. A higher perveance thus indicates that the emitted electron beam is more heavily influenced by space-charge effects. In such a system, the voltage that must be applied to the focus electrode in order to completely cut off the beam current becomes very large. It would thus be beneficial to implement the gun header using a stacked ceramic structure that can support the various elements of the electron gun and also provide a high voltage standoff to support the high voltages necessary to sustain operation at a high perveance. It would also be beneficial to mechanically adjust the perveance of the electron gun and optimize the magnetic flux at the cathode to achieve improved beam focusing.